There are many technologies for water/wastewater treatment with practically no limit to water quality achievable when treating a majority of the existing water/wastewater streams. Biological treatment is the most widely used technology. It utilizes metabolism of microorganisms to remove organic matter, as well as other dissolved nutrients including nitrogen and phosphorus. Biological mass (biomass) is also known to adsorb heavy metals, suspended solids, and other sorts of contaminants which do not undergo biological degradation. Biomass is separated from the treated liquid, thus allowing for discharge of treated water (effluent) and disposal of the excess of the biomass (sludge). Depending on the quality of the water for treatment (influent), two biological treatment methods are typically used separately or in combination. A first is anaerobic treatment, which does not require aeration (addition of dissolved oxygen). The other is aerobic treatment, utilizing dissolved oxygen in the biological treatment.
For concentrated wastewater streams, anaerobic treatment is commonly used to achieve partial degradation of the contamination. Although aerobic treatment consumes more energy than anaerobic treatment, aerobic treatment is often used to achieve a more rapid and complete removal of the organic pollutants. The activated sludge method is an example of an aerobic biological treatment for municipal wastewater containing a relatively low level of organic impurities using biomass mixed with the treated liquid.
Recent development in wastewater treatment technology have demonstrated integration of a filtration membrane (micro or ultra) with activated sludge or anaerobic treatment provides effective method of sludge separation and process control, achieving more efficient treatment. Such a combination is called a Membrane Bioreactor (MBR). However, the effluent produced by biological treatment and microfiltration is insufficient for significant number of uses as the effluent contains bacteria, viruses, and residual amounts of organic and inorganic contaminants. Therefore additional treatment, such as chemical disinfection, UV disinfection, ion exchange, sorption, etc. is common. Because of limitations in treatment efficiency, these technologies are often used in combination, resulting in high treatment cost.
Reverse Osmosis (RO) technology is another commonly used process which provides high treatment efficiency. RO membranes effectively remove suspended solids (including viruses and bacteria), often with higher efficiency and reliability than MBR. RO membranes also remove inorganic matter (including dissolved salts, thus providing softening effect). RO can also remove high molecular weight dissolved organics, which is typically a main fraction of the biological treatment effluent.
However, conventional implementations of both MBR and RO technologies have significant drawbacks resulting in high treatment cost. For example, MBR requires a long retention time to ensure efficient nitrogen removal. This long retention time translates into large footprint and higher capital cost. Also, conventional MBR implementations require a large number of units of mechanical equipment, including hydraulic pumps, blowers, compressors, vacuum pumps, etc. This again increases capital cost and maintenance cost, and raises reliability concerns.
Deficiencies in conventional RO implementations include high-energy consumption and high pretreatment cost. Another problem with treating MBR effluent by RO is bio-fouling of RO membranes. Controlling bio-fouling by disinfection is difficult due to the fact that oxidizing biocides may attack the membrane material and adversely affect membrane performance. Disinfection also typically entails a use of chemicals which require special permits and adds operational complexity
In view of recent trends in environmental/health regulations, as well as greater public awareness of the importance of clean water, decentralized small-scale treatment technologies are expected to become more important. Generally, when scaling the typical applications down, the cost of each volumetric unit of the treated water increases exponentially. More particularly, the operational difficulties, permitting, and concomitant costs described above have thus far limited application of these treatment technologies to large treatment plants of a size of the POTW (publicly owned treatment works). Furthermore, systems designed to overcome the constraints typical of smaller scale systems may also prove to be cost competitive for implementations at the POTW scale.